I tried the same things, flywheel weight long pipe etc, but ended up with a 250 which as Shirty ably demonstrated in scotland is plenty enough for anyone.
I never advise a slow throttle as it means a big twist to get full revs and doesnt help with throttle control.
Yes a 10 front sprocket should be fine, i think a 9 is a bit too small. If its on its way and youre already thinking of softening the power that should tell you something.